Actually, there is a better way to eliminate this CPU load without disabling the ADC driver and losing the ability to access the SOC temperature. The Linux thermal management periodically polls the SOC temperature, and you can disable it by adding the following line to /etc/rc.local:
echo disabled > /sys/devices/virtual/thermal/thermal_zone0/mode
Details/background:
The A20 ADC driver is very inefficient: it seems to be using a busy loop (as mentioned here: https://linux-sunxi.narkive.com/1UetD5rh/bug-report-kworker-issue-for-mailline-kernel-4-12)
The Linux thermal management periodically polls the SOC temperature (runs the ADC) at around 0.9Hz. I presume this would be useful for controlling the CPU fan, but probably not applicable to most A20 systems.
Disabling this polling (as above) removes the CPU load, while still allowing inefficient one-off reads of the CPU temperature, as well as other ADC usage.
mikhailai got a reaction from Bernie_O in K-worker problem on A20 based boards